Huntington disease (Huntington's disease; HD) is a neurodegenerative disorder caused by an autosomal dominant mutation in the HTT gene—an expansion of the trinucleotide repeat located on exon-1 of HTT to 36 or more polyglutamine (CAG) repeats—leading to the production of mutated Huntingtin (mHTT). This evidence concerns the gene HTT and Huntington disease.